The invention relates to a circuit arrangement for an a-c series motor with a switching device for changing from motor to braking operation in general and more particularly to an improved arrangement of this nature.
In a known circuit, a first switch contact is arranged between the armature winding and the field winding of the motor and a second switch contact across the series circuit consisting of the first switch contact and the armature winding, and a third switch contact across the series circuit consisting of the first switch contact and the field winding, a capacitor and a charging diode in series being associated with the a-c series motor.
In motor operation, the first switch contact is closed and the second and third switch contacts are open. When changing to braking operation, the motor is separated from the supplying a-c network and the first switch contact is opened and the second and third switch contact are closed. The armature and the field windings are thus in a closed circuit. The residual field in the field winding induces an electromotive force in the armature winding which causes a braking current to flow which decelerates the motor.
At the beginning of such a self-excited braking operation, there is not always a field with an unequivocally defined excitation. In the worst case, the electromotive force induced in the field winding is so small that no current is built up in the braking circuit and the motor cannot be braked at all. This can happen particularly if the commutator of the motor is dirty.
U.S. Pat. No. 3,673,482 describes a circuit arrangement in which a capacitor and a diode in series are connected directly between the two a-c leads during motor operation. In motor operation, the capacitor is charged from the a-c voltage via the diode. A series resistor is required for limiting the charging current for the capacitor. At the beginning of the braking operation, the field winding is externally excited for a short time by a discharge of the capacitor. The field winding is thereby excited in an unequivocally defined manner and induces an electromotive force in the armature winding which drives a sufficiently large braking current.